The present invention concerns liquid crystal display cells and, more particularly, a novel process for providing homeotropic surface alignment in liquid crystal display cells.
In many forms of liquid crystal display cells, it is desirable to provide homeotropic surface alignment at the boundaries between the liquid crystal material and the electrodes and surfaces of the cell-defining substrates. For example, in a reflective dichroic liquid crystal display utilizing the cholesteric-nematic transition, it is known that providing a homeotropic surface alignment results in a display cell which not only has a relatively low threshold voltage for the desired display material transition, but which also provides a more uniform appearance to the observer in the quiescent state, with substantially no adverse effects due to the absorption of stray polarized light by the cell. In this type of cell, the "focal conic texture", which is present immediately after a display segment has been deactivated, is provided with an absorbance more nearly approaching the absorbance due to the homeotropic quiescent state, resulting in a display of improved overall appearance.
It is equally as well known that the homeotropic boundary condition can be achieved by absorption of a surfactant on the interior surfaces of the display device cavity, or by the addition of surfactants to the liquid crystal material, such that, when the cell is filled, the "polar" surfactant molecules absorb on the surface of the cell interior walls to achieve the desired homeotropic boundary condition. However, the use of additives in the liquid crystal material tends to increase the conductivity of the material, and hence increases the current and power required for switching the optical states of the device. In some applications, use of certain surfactants, such as silane coupling agents, may be acceptable. However, in certain applications a display cell having a high degree of hermeticity is required, which high degree of hermeticity can only be achieved by use of a glass frit peripheral seal. Providing such a seal requires that the glass frit seal material be brought to a temperature of between about 500.degree. C. and about 550.degree. C., which is well above the temperature, on the order of 250.degree. C., at which available surfactants decompose. It was hitherto impossible to reproducibly achieve a homeotropic alignment in cells which must use glass frit seals for achieving a high degree of hermeticity. A process by which homeotropic alignment can be achieved in liquid crystal display cells having glass frit seals, and further capable of large scale production, is therefore highly desirable.